The rating as to how attractive or hot a girl is. It generally goes in the order of:
Butt
Breasts
Face
Some people also rate the rest of their body.
So if someone tells you a girl is a 7-6-7, they mean her butt is a 7 out of 10, her breasts are a 6 out of 10, and her face is a 7 out of 10 also.